1. FIELD OF THE INVENTION
This invention relates to a method for producing a nonvolatile memory device having an excellent memory retention characteristic.
2. DESCRIPTION OF THE PRIOR ART
In a conventional nonvolatile memory device of metal-nitride-oxide-semiconductor type (nonvolatile memory device of MNOS type), i.e. nonvolatile memory device using a double layer of silicon nitride and silicon oxide as a gate insulating film, aluminum is generally used as gate metal. This is partly because aluminum is easy to work and because the formation of the film thereof is easy so that the process of using aluminum was early realized, but mainly because the process using polycrystalline silicon instead of aluminum could not produce a memory device having a satisfactory memory retention characteristics. Accordingly, the nonvolatile memory device of MNOS type has not yet come to have a silicon gate whereas the standard MOS memory device has had a silicon gate.
Namely, in the process using aluminum as gate metal, the steps involving high-temperature heat treatments such as thermal diffusion treatments for forming a source and a drain are performed before the step of forming a nitride-oxide double film. Accordingly, the state immediately after the formation of the silicon nitride film is kept intact and the quality of the formed film remains almost invariable, so that there is no risk of the memory retention characteristic being adversely affected.
The memory retention in the nonvolatile memory device is defined as the period of time for which a supplied signal can be memorized. In a MNOS type nonvolatile memory device, signals are memorized through the accumulation of electric charges in the interface between the silicon nitride film and the silicon oxide film or in the silicon nitride film by the application of a high voltage to the gate electrode. The memory retention property is the property of retaining the charges in the interface or the silicon nitride film.
There are three parameters which are considered to mainly deteriorate the memory retention property. They are:
(1) the trap density and the trap depth in the interface between the silicon nitride and the silicon dioxide films or in the silicon nitride film,
(2) the thickness of the silicon dioxide, and
(3) the surface state in the interface between the silicon substrate and the silicon dioxide film.
Of these parameters, the first is concerned with the rate of loss of the stored charges due to thermal excitation, and the second and the third affect the rate of the escape of the stored charges into the surface of the silicon substrate through back tunneling. As described before, in the case where aluminum is used as a gate electrode, a high-temperature heat treatment is not performed after the formation of the silicon nitride film so that the above parameters are all kept invariable, therefore, the memory retention property is never deteriorated.
In order to improve the packing density and the occupying rate in a MNOS type nonvolatile memory device and to further the performance thereof, the memory device must be fabricated by using as gate metal polycrystalline silicon, refractory metals such as Mo, W, Ta, Ti, Cr, Ni, etc. or their alloys or silicon compounds through a self-aligning process.
In the self-aligning process using polycrystalline silicon etc. for a gate electrode, a gate is formed and then a source and a drain are formed with the gate being used as a mask, therefore a high-temperature heat treatment is necessary after the formation of the gate electrode.
In the conventional fabrication process using polycrystalline silicon for gate electrode, such a high-temperature heat treatment after the formation of the gate electrode is usually performed in the atmosphere of nitrogen or oxygen. According to this conventional process, the above-mentioned parameters are varied and therefore the memory characteristic is always deteriorated. Thus, according to the conventional process, it was very difficult to fabricate a nonvolatile memory device having a high quality using polycrystalline silicon as gate metal.